This application claims priority to Italian Application Serial Number 2002A000803, filed Sep. 16, 2002.
1. Field of the Invention
The present invention relates to current-reference circuits. More particularly, the present invention relates to temperature-compensated current-reference circuits.
2. The State of the Art
In integrated circuit applications such as flash memory, EEPROM, and others, certain circuits require a constant current that is independent of variations in temperature and supply voltage.
Numerous techniques exist for designing current references to be unaffected by supply-voltage and temperature variations. One way to generate a current reference that is robust with respect to supply-voltage variation but sensitive to temperature variation is to employ two current mirrors and a resistor as shown in FIG. 1. The current through p-channel MOS transistor 10 is mirrored through p-channel MOS transistor 12. The current through n-channel MOS transistor 14 is mirrored through n-channel MOS transistor 16, having resistor 18 coupled between its source and ground.
The circuit of FIG. 1 has a current variation of up to about 30% as a function of temperature. For circuits of the type shown in FIG. 1, the current generated is equal to:
I=n*Ut*ln(M)/R
if the transistors are in weak inversion and
I=(2/Kn*R2)*"psgr"(I)
if the transistors are in strong inversion. In both cases the current is independent of the supply voltage but temperature variation is uncompensated.
Another way to provide a current reference is to employ a resistor and a bipolar transistor as shown in FIG. 2 to generate a current that is proportional to both absolute temperature and the temperature coefficient of the resistor.
P-channel MOS transistors 20 and 22 have their gates driven from the output of operational amplifier 24. PNP bipolar transistor 26 has its emitter coupled to the drain of p-channel MOS transistor 20 and its base and collector coupled to ground. PNP bipolar transistor 28 has its emitter coupled to the drain of p-channel MOS transistor 20 through resistor 30 and its base and collector coupled to ground. One input of operational amplifier 24 is coupled to the drain of p-channel MOS transistor 20 and the other input of operational amplifier 24 is coupled to the drain of p-channel MOS transistor 22.
In the circuit of FIG. 2, the current is given by:
I=(Ut/R)*ln(N)
In order to provide temperature compensation, the temperature coefficient of the resistor must be opposite to Ut.
The present invention provides a temperature-compensated current reference using only a MOS transistor and polysilicon resistor of the same type.